The present invention relates to the operation of a Polymer Process with specific example applied to a Polypropylene Process (PP). PP in this example comprises of nine operation areas—the catalyst preparation area (Cat Prep), reactors (RX), recycle gas compressors, recycle gas recovery system, the dryers, two granule areas and two extruders system. In particular, the present invention relates to determining when the process is deviating from normal operation and automatic generation of notifications isolating the abnormal portion of the process.
Polypropylene process (PP) is one of the most important and widely used processes for polymerizing propylene to produce polypropylene. Polypropylene is then used as intermediate materials in producing plastic products such as milk bottles, soft drink bottles, hospital gowns, diaper linings etc. The PP is a very complex and tightly integrated system comprising of the catalyst preparation unit, reactors, recycle gas compressors, recycle gas recovery system, the dryer, granule systems and two extruders. FIG. 23 shows a typical PP layout. The PP process employs catalysts in the form of very fine particles mixed with cold oil and grease to form a very thick and paste—like mixture. The thick and paste-like property of the catalyst mixture makes it difficult to pump catalysts into the reactors, thus makes the catalyst system prone to plugging problems. The catalyst mixture and the fresh monomer feed (Propylene or C3=) along with the co-monomer (Ethylene) are fed into two large reactors (RX 1 & RX 2) in series. In the reactors, when the monomers are in contact with the catalysts, a very exothermic reaction occurs and polymer granules are formed in the reactor slurry. To remove the heat generated by the reaction, cooling water is continuously pumped around the reactor jackets to maintain the reactor temperature at a desired target. In each of the first two reactors, there is a big pump continuously circulating the polymer slurry to prevent the formation of chunks. Depending on the product grades, the PP has two distinct reactor configuration modes—One configuration mode utilizes two reactors (RX 1 & 2) in series, while the other mode requires a third reactor (RX 3) in series with the first two reactors. The polymer slurry exiting the reactors is pumped into the separators where un-reacted monomers are removed and sent to the monomer recovery system before recycling back to the reactors. The polymer granules are fed into the dryer system where any last trace of monomers is removed, any trace of catalyst residues is steam stripped and the granules are dried off. The dry polymer granules are sent to the granule system where they are blended with additives and sent to the two extruders for pelletization. The polymer pellets are then sent the storage system or to the load out system.
Due to the complicated dynamic nature of the PP, abnormal process operations can easily result from various root causes that can escalate to serious problems and even cause plant shutdowns. These operations can have significant safety and economic implications ranging from lost production, equipment damage, environmental emissions, injuries and death. A primary job of the operator is to identify the cause of the abnormal situation and execute compensatory or corrective actions in a timely and efficient manner.
The current commercial practice is to use advanced process control applications to automatically adjust the process in response to minor process disturbances, to rely on human process intervention for moderate to severe abnormal operations, and to use automatic emergency process shutdown systems for very severe abnormal operations. The normal practice to notify the console operator of the start of an abnormal process operation is through process alarms. These alarms are triggered when key process measurements (temperatures, pressures, flows, levels and compositions) violate predefined static set of operating ranges. This notification technology is difficult to provide timely alarms while keeping low false positive rate when the key measurements are correlated for complicated processes such as PP.
There are more than 450 key process measurements, which cover the operation of a typical PP. Under the conventional Distributed Control System (DCS) system, the operator must survey this list of sensors and its trends, compare them with a mental knowledge of normal PP operation, and use his/her skills to discover the potential problems. Due to the very large number of sensors in an operating PP, abnormalities can be and are easily missed. With the current DCS based monitoring technology, the only automated detection assistance an operator has is the DCS alarm system which is based on the alarming of each sensor when it violates predetermined limits. In any large-scale complex process such as the PP, this type of notification is clearly a limitation as it often comes in too late for the operator to act on and mitigate the problem. The present invention provides a more effective notification to the operator of the PP.